Typically, marking particle transport devices are used to transport developers from a storage area to a photographic element. The photographic element is usually an image carrier, which is stretched over one or several rollers, for example an endless band, which already has a latent image thereon. The developer is a mixture, composed of at least magnetic carrier particles and non-magnetic marking particles, in which the marking particles attach to the magnetic carrier particles through triboelectric charging. As soon as the non-magnetic marking particles attach to the latent image on the photographic element, the magnetic carrier particles are removed again. Finally, the image composed on the marking particles is transferred to a paper-like or endless print substrate, and the non-magnetic marking particles are fixed to such substrate in a fixing station through pressure and heat.
To transfer such a developer mixture to the substrate, typically a so-called magnetic brush is used, which is known in the state of the art. For example, in U.S. Pat. No. 4,357,103, such a magnetic brush developer device is described. The magnetic brush developer device shown there displays a non-magnetic cartridge, which situated near the photographic element and slowly rotates. The developer is transported to this cartridge. The developer attaches to the cartridge because there is a multi-polar magnetic roller inside the cartridge, which attracts the magnetic carrier particles and presses on the surface of the cartridge. In this specification, a further magnet is located opposite the photographic element, which reduces the magnetic field of the multi-polar magnet roller in the contact area of the developer and the photographic element and thereby eases the transfer of the developer.
In the European disclosure specification EP 929 006 A2, a cartridge containing a multi-polar magnet roller is shown, in which the cartridge and the multi-polar magnet roller rotate at the same time. In addition, there has also been added an electromagnet on the opposite side of the photographic element, which is controlled in such a way that the desired magnetic field is formed in the contact area between cartridge and photographic element. A comparable device can also be seen in the Japanese specification JP 57118269 A2.
In the Japanese specification JP 57078575 A2, two multi-polar magnet rolls are used, which are situated on both sides of a photographic element, in order to bring the magnetic developer on to the photographic developer. In this case, the multi-polar magnet rolls turn in an opposite direction to each other.
In order to gain more influence on the spatial distribution of the magnetic forces in a typical developer station or other device for the controlled transport of magnetic particles or mixtures of magnetic and non-magnetic particles, in order to stem or compress the developer for example or to remove it from a surface, it would be advantageous to have a stationary, i.e., constant in its strength, magnetic field.
If a force from the magnet roller is not wanted locally, then an additional magnet must be added. The field is then calculated from the absolute value of the vector totals of the magnetic inductions. This overlapping of a stationary magnetic field with a rotating multi-polar field is alternately constructive and destructive and therefore, the absolute value is not constant. It behaves in a similar way if the second magnet is a roller, which turns in an opposite direction. In this instance, it is not possible to create a stationary magnetic field.